1. Technical Field
The present invention relates to a fixing device, and more particularly, to a fixing device for use in an image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of these features, an image is formed by attracting developer or toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium with heat and pressure.
In general, a fixing device employed in electrophotographic image formation includes a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip. As a recording medium bearing a toner image thereupon enters the fixing nip, heat from the fuser member causes the toner particles to fuse and melt, while pressure between the fuser and pressure members causes the molten toner to set onto the recording medium.
To date, some fixing devices employ a small-sized, thin-walled fixing roller that exhibits an extremely low heat capacity. Although allowing a fast, energy-efficient fixing process that can process a toner image with a short warm-up time and reduced energy consumption, those fixing devices are susceptible to variations in fixing performance due to insufficient heating of the low-heat capacity equipment, from which a substantial amount of heat is dissipated as the recording medium passes through the fixing nip.
To prevent variations in fixing performance, one approach is to design a fuser roller with its circumferential length longer than a shorter edge of a recording sheet accommodated in the fixing device, such as A4-size copy paper. Such arrangement allows the recording sheet to pass through the fixing nip in a shorter period of time than that required for one rotation of the fuser roller, thereby enabling uniform heat distribution from the fuser roller along the length of the recording sheet.
Although generally successful for its intended purposes, the method described above has a limitation in that it cannot effectively prevent variations in fixing performance during sequential processing of multiple recording sheets through the fixing nip, or upon a change in imaging speed measured in terms of number of recording sheets per unit time.
Where heater power supply is controlled independently of conveyance of the recording sheet, a delay or difference in time may arise between when the recording sheet enters the fixing nip and when the heater is activated to heat the fuser member. Such a lack of synchronization between heater activation and entry of the recording sheet into the fixing nip results in variations in the amount of heat applied to the recording sheet, leading to variations in fixing performance.